Ball joints are often used in the front wheel suspension of automotive vehicles. The ball joints permit steering of the wheels while accommodating changes in angle between the wheel and the suspension members. In conventional ball joints, a pin on which the wheel is mounted carries a ball, which is rigid with the pin, the ball being received in a cup. During both rotation and pivoting of the pin, the surface of the ball slides over a lining of the cup. Thus, since both modes of motion take place between the same sliding surfaces, they are subject to the same level of frictional resistance. However, while relatively high friction is of some advantage for the pivoting movement, in order to give a damping effect to the suspension, low friction rotary movement is often desirable in order to reduce the forces required for steering.
British Patent Specification No. 758805 discloses a ball joint having a ball element which is rotatably mounted on a stud. A thrust bearing comprising balls is provided to withstand axial loads in one direction. The stud is a sliding fit in the ball element, and radial and tilting loads are transmitted by direct contact between the stud and the ball element. This direct contact gives rise to undesirable friction resisting relative rotation between the stud and the ball element.